zero_indices.push_back(i);
          }
        }
      } else {
        any_gradient_nonzero = true;
        Gradient* new_gradients = nullptr;
        if (grad_it->second.size() == 1) {
          new_gradients = grad_it->second.at(0);
        } else {
          new_gradients = vspace.AggregateGradients(grad_it->second);
        }

// Runs `model` maybe wrapped in a function call op. This can be thought as
// being equivalent to the following python code.
//
// if use_function:
//   outputs = tf.function(model)(inputs)
// else:
//   outputs = model(inputs)
Status RunModel(Model model, AbstractContext* ctx,
                absl::Span<AbstractTensorHandle* const> inputs,
                absl::Span<AbstractTensorHandle*> outputs, bool use_function);